Several types of material handing devices are known. One type of material handling device, known as a balancer, consists of a motorized take-up pulley, a rope which wraps around the pulley when the pulley turns, and an end-effector which is attached to the end of the rope. The end-effector has components that connect to the load being lifted. The rotation of the pulley winds or unwinds the rope and causes the end-effector to lift or lower the load. In this class of material handling system, an upward force in the rope exactly equal to the gravity force of the object being lifted is generated by an actuator; the rope tension is equal to the weight of the object. Therefore, the only force the operator must impose to maneuver the object is the force necessary to overcome the object's inertia. This force can be substantial if the mass of the object is large. Therefore, the ability to accelerate or decelerate a heavy object is limited by the operator's strength.
There are two ways of creating a force in the rope so that it is exactly equal to the object weight. First, if the system is pneumatically powered, the air pressure is adjusted so that the lift force equals the weight of the load. Second, if the system is electrically powered, the correct voltage or current (depending on the control circuitry) is provided to an amplifier to generate a lift force that equals the load weight. These types of systems are not suited to maneuvers in which objects of varied weights are lifted. This is true because each object requires a different bias force to cancel its weight force. This annoying adjustment can be done either manually by the operator or electronically by measuring the object weight.
For example, the BA Series of balancers made by Zimmerman International Corporation work based on the above principle. The air pressure is set and controlled by a valve to maintain a constant load balance. The operator has to manually reach the actuator and set the system to a particular pressure to generate a constant tensile force on the rope.
The LIFTRONIC System machines made by Scaglia of Italy also belong to the family of balancers, but they are electrically powered. As soon as the system grips the load, the LIFTRONIC machine creates an upward force in the rope which is equal and opposite to the weight of the object being held. These machines may be considered superior to the Zimmerman BA Series balancers because they have an electronic circuit that balances the load during the initial few moments when the load is grabbed by the system. As a result, the operator does not have to reach the actuator on top and adjust the initial force in the rope. In this system, the load weight is measured first by a force sensor in the system. While this measurement is being performed, the operator should not touch the load, but instead should allow the system to find the object's weight. If the operator does touch the object, the force reading will not be correct. The LIFTRONIC machine then creates an upward force in the rope which is equal and opposite to the weight of the object being held.
Balancers of the kind described above do not give the operator a sense of the force required to lift the load. Also, only the weight of the object is canceled by the rope's tension. Moreover, such balancers are generally not versatile enough to be used in situations in which load weights vary.
Another class of machines is similar in architecture to the machines described above, but the operator uses an intermediary device such as a valve, pushbutton, keyboard, switch, or teach pendent to adjust the lifting and lowering speed of the object being maneuvered. For example, the more the operator opens the valve, the greater the speed generated to lift the object. With an intermediary device, the operator is not in physical contact with the load being lifted, but is busy operating a valve or switch. The operator does not have any sense of how much he/she is lifting because his hand is not in contact with the object. Although suitable for lifting objects of various weights, this type of system is not comfortable for the operator because the operator must focus on an intermediary device (i.e valve, pushbutton, keyboard, or switch). Thus, the operator pays more attention to operating the intermediary device than to the speed of the object. This makes the lifting operation rather unnatural.